Intramolecular ligand-ligand hydrophobic interactions of ternary complexes of Zn(II)ion with amino acids

Stability constants of ternary complexes Zn(Ltyr)(Aa)were determined by pH potentiometric titrations in water,φ=020?040and 060 dioxanewater solvents,where Ltyr-=Ltyrosinate,Aa-=Lvalinate,Ltryptophanate and Lphenylalaninate (t=25℃,I=01mol/L NaClO4).The relative stability of the ternary complexes is defined by δK {=ΔlogKZnZn(Ltyr)(Aa)-logKZnZn(Ltyr)-logKZn-ΔlogK(s)}.The results show that,compared with the complex Zn(Ltyr)(Lala) with Lalaninate,the complexes with the amino acids have larger δK values,indicating that there are extra stabilizing effects in the ternary complexes.This may be mainly attributed to the intramolecular hydrophobic or stacking interactions between the phenyl group of Ltyr and the side chains of other amion acids in the complexes,and the interactions vary with the side chains of the amino acids or solvent polarity.